The invention relates to an internal combustion engine, particularly a supercharged engine, with a divided exhaust flow and particularly relates to control over the operation of the intake and exhaust valves to the cylinders.
The use of turbocharged engines, that is to say engines in which supercharging is carried out by means of a compressor which is driven by an exhaust-driven turbine, is becoming increasingly common within automotive technology. At high power, a greater charge can as a result be utilized than in a conventional engine. At full throttle, however, poor volumetric efficiency is obtained as a consequence of a negative pressure difference between the inlet and the outlet during the period in which inlet valves and exhaust valves are open at the same time. This means that it is not possible to empty the cylinder completely during the exhaust stroke, as a result of which the desired quantity of new air cannot be supplied to the cylinder either. Another consequence is that the exhaust flow at low engine speed is insufficient to drive the turbine effectively, with the result that the charging pressure is not optimal.
By utilizing the known principle of the divided exhaust period according to patent specification GB 2 185 286, the operation of the exhaust turbine can be improved by virtue of the fact that the exhaust flow is divided up so that only the high-pressure flow goes to the exhaust turbine while the low-pressure flow is conducted past the exhaust turbine. This is achieved by virtue of the fact that there are at least two exhaust valves in each cylinder, which open differently and feed separate exhaust manifolds. In this way, better emptying of the cylinder is made possible.
During the exhaust stroke of the piston, the intake valve opens before the piston has reached its top dead centre position, at the same time as the second exhaust valve (the last opened) is still open and is on the point of being closed after the piston has passed its top dead centre position. There is therefore a valve overlap around the top dead centre position of the piston, in which one exhaust valve and one intake valve are open at the same time, and in which the pressure in the exhaust system is low. If this valve overlap is too great, a large part of the air fed in by the compressor can at full throttle xe2x80x9cleak outxe2x80x9d via the open exhaust valve and will therefore not be burned in the cylinder. The result is an imbalance in the mass flow between the compressor and the turbine, which results in the turbine not being capable of making the compressor deliver sufficient charging pressure. This in turn results in the torque being impaired. On the other hand, if the valve overlap is too small, residual gases will remain in the cylinder, with the result that the quantity of combustion air is reduced with the attendant reduction in power.
The aim of the invention is to produce a turbo-engine which has improved characteristics within a greater engine speed range at full load.
The aim of the invention is achieved by producing an internal combustion engine as indicated above wherein the valve open condition overlap around the top dead center position of the piston is adjusted by valve operation controlling or timing elements to synchronize the operation of those valves so that the length of the open condition overlap increases with engine speed when the engine is driven at high load and particularly causes the intake valve to open increasingly early before the top dead center position of the piston while one of the exhaust valves closes increasingly late after the top dead center position of the piston.
By means of the chosen embodiment, it is possible to control the gas flow through the engine so that exhaust gases are allowed out without an unnecessarily great quantity of air fed in being able to xe2x80x9cleak outxe2x80x9d via an exhaust valve which is still open. This leads to more effective filling of the cylinder and as a result an increased mass flow of exhaust gases, which in turn increases the possibility of increasing the torque at low engine speed by means of a higher charging pressure.
The synchronization between the valves can advantageously be such that, as the engine speed increases, the intake valve opens increasingly early before the top dead centre position of the piston while the second exhaust valve closes increasingly late after the top dead centre position of the piston. In this way, the position of maximum opening can be reached when the piston is at or very close to the top dead centre position.
The relationship between the engine speed and the length of the overlap period can suitably be exponential.
Further features and advantages of the solution according to the invention emerge from the description and the other patent claims.